An algae scrubber is a water filtering device (not to be confused with a scrubber pad used to clean glass) which uses light to grow algae; in this process, undesirable chemicals are removed from the water. Algae scrubbers have allowed saltwater and freshwater aquarium, or, pond hobbyists the ability to operate their tanks the way that oceans and lakes operate: using natural filtration in the form of primary production.
Both iron fertilization and ocean nourishment are techniques that boost the primary production of algae in the ocean, which consumes massive amounts of nutrients and CO2. It is this same consumption of nutrients that algae perform in an aquarium or pond. An algae scrubber filters water by moving water rapidly over a rough, highly illuminated surface, which causes algae to start growing in large amounts. As the algae grow, they consume nutrients such as nitrate, phosphate, nitrite, ammonia, ammonium and even metals such as copper from the water. These nutrients are normally a problem in aquariums and ponds because they cause nuisance algae to grow, and also because they cause sickness and/or other problems in aquarium fish, invertebrates and corals. An algae scrubber allows algae to grow, but the algae grow inside the filter instead of in the aquarium or pond. This removes excess nutrients (scrubs the water), diminishing nuisance algae in the aquarium or pond . Nuisance algae in the aquarium or pond are not to be confused with the desired algae in the algae scrubber filter itself. The algae that grow in the algae scrubber can then be removed, or fed back to the livestock.
Algae scrubbers are used in both saltwater and freshwater, and remove nuisance algae of multiple types: cyano or slime, bubble, and hair.
Generally, and except for specific continuous-filtering or continuous-cultivating versions, algae scrubbers require the algae to be removed ("harvested") periodically from the scrubber. This removal of algae has the effect of removing undesired nutrients from the water because the algae used the nutrients in order to grow. The algae is generally removed either:
- Every 7 to 21 days, or
- When it is black, or
- When it fills up the scrubber, or
- When it starts letting go, or
- When nutrients start to rise in the water.
For waterfall versions, the screen is removed from the pipe and cleaned in a sink with running water. The pipe is removed also, and the slot is cleaned with a toothbrush, to remove any algae that have grown up into it. After the algae are removed, the screen and pipe are put back in the scrubber.